[Bugfix][KV Offload] Namespace auto cache dtype by effective dtype

[jongukc]

Purpose

Persistent native KV-offload tiers currently fingerprint the unresolved
cache_dtype argument. With the default cache_dtype="auto", FP16 and BF16
engines for the same model therefore share a filesystem or object-store
namespace even though their KV bytes are incompatible. Because both formats
have the same page size, a BF16 engine can accept FP16 pages and silently return
incorrect output after restart.

This change resolves only the auto case to model_config.dtype before
constructing OffloadingModelConfig. Explicit cache formats such as FP8 remain
unchanged because they may share a backing torch.dtype while using
incompatible representations.

Duplicate-work check

I searched open and closed issues and PRs for FileMapper, persistent native
offload, cache_dtype=auto, FP16/BF16, and cache namespaces. I found no matching
fix. #49261 and #49266 add model-revision identity to the same namespace, but
they do not resolve the literal auto dtype. This finding is independent and
persists within one model revision. #48414 fingerprints an opt-in canonical
layout, and #49413 concerns offload-event routing; neither changes cache dtype
identity.

Test plan

.venv/bin/python -m pytest -q \
  tests/v1/kv_offload/test_factory.py \
  tests/v1/kv_offload/test_file_mapper.py

.venv/bin/python -m pytest -q tests/v1/kv_offload

.venv/bin/pre-commit run --files \
  vllm/distributed/kv_transfer/kv_connector/v1/offloading/config.py \
  tests/v1/kv_offload/test_factory.py

git diff --check origin/main...HEAD

The model-level replay uses facebook/opt-125m, V2, FP16/BF16, a fixed
PYTHONHASHSEED, and a persistent filesystem tier. It runs cold BF16, populates
the tier with FP16, and then starts BF16 against the same root.

Test results

Tested on current upstream main at d6dbdb9b0.

• Focused factory and mapper tests: 40 passed.
• Complete KV-offload suite: 388 passed.
• All applicable pre-commit hooks passed, including Ruff and mypy.
• git diff --check passed.
• The fixed replay created distinct float16 and bfloat16 namespaces.
• The BF16 replay JSON, including token IDs and chosen-token logprobs, was
  byte-identical to the cold BF16 baseline.

Before the fix, the same replay loaded FP16 pages into BF16 and changed the
greedy continuation from Paris... to the first to be a great person....

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[orozery]

Thanks for the fix @jongukc !

[jongukc]

@orozery It seems that the failed CI jobs are unrelated to this patch.
(There was a network error during apt command.)
Could you retry them when convenient?

[orozery]

@orozery It seems that the failed CI jobs are unrelated to this patch. (There was a network error during apt command.) Could you retry them when convenient?

I usually just click "update branch" until the flaky CI tests pass.
It seems that currently there is a CI jobs that fails deterministically (buildkite/ci/v1-sample-plus-logits).
I think it's best if we watch the current main and wait for it to stabilize, then "update branch" to re-trigger the CI.